This research will investigate the effects on the craniofacial complex of the altering the length of the muscles of mastication in an animal model, the rhesus monkey, Macaca mulatta. Two sizes of bite opening appliances will permit study of (1) lengthening of masticatory muscles, and (2) reducing the length of the masticatory muscles following an initial adaptation to increased length. The primary focus of the study will be to investigate the internal adaptations of the muscles through the use of techniques to measure voluntary bite force in concert with EMG and maxillary bone strain, involuntary bite force following muscle stimulation, whole muscle contractile properties, as well as morphological features such as fiber length, sarcomere length, fiber size, fiber type and other histological and and histochemical characteristics. A secondary focus of this study will be to investigate altered patterns of facial bone strain that result from altered function following placement of a bite-opening appliance and concurrent changes in growth and remodeling of the facial skeleton. It is hypothesized that internal adaptations of the masticatory muscles will be manifested by shifts in fiber length, length-tension relationships, and patterns of EMG activity when biting at different gapes. Skeletal adaptations result from increased upward bending of the midface and altered condylar position following appliance placement. The results of these studies have a direct impact on related clinical problems of muscle and skeletal adaptations when muscle length is altered following various surgical, prosthodontic, or orthodontic procedures.